A fabrication shop's functions can overlap a bit with those of machine shops, but fabricators usually focus on building structures and devices from steel and aluminum, rather than on machining parts. Hence, fabrication shops perform operations that include welding and assembly, press-brake forming, laser cutting, punching and stamping, and rolling and forming. In the medical area, metal fabrication can handle everything from building a mundane but necessary medical carts that carry life-saving equipment to the cutting and assembly of intricate medical devices such as endoscopic instruments.

Equipment cart goes high tech

Hospitals are a good example of medical facilities that make good use of carts and cabinets fabricated from metal. “It's common for a lot of medical-device manufacturers to suddenly realize at the end of the design cycle that they have forgotten, of all things, a cart for their new device,” says NBD Product Development Manager Mark Collins of HUI Mfg., in Kiel, Wisc., (huimfg.com). “When off-the-shelf units are unsuitable because they are shaped wrong or too flimsy, many firms turn to custom-fabrication companies like ours. Most customers have similar design requirements: A medical-device cart must be stable, hold a lot of expensive equipment, maneuver easily, and show off a cool, curvy style.”

Because real estate at medical facilities is usually at a premium, most places want the smallest footprint and the lightest weight cart possible, says Collins. “Of course, there are always trade-offs. For example, a narrow wheel base necessitates a rigid cast base. That's not necessarily bad because carts get abused during constant use and when they are not durable enough, they can collapse. A cast base would seem to fit the bill here, but it does have drawbacks. These include tooling and development times, inconsistent surface finishes, larger annual build volumes to offset tooling investment, and the possibility of catastrophic failure due to crack propagation in a brittle material,” he says.

To get around this problem, Collins' company devised a metal-fabrication method that provides all the positive aspects of a cast base without its drawbacks. The method entails stacking layers of mild steel around the perimeter of the base and welding them together. The thickness of the material depends on the weight requirements of the base and the selection of the rubber molding around the outside perimeter. A small space or slot in the side wall extending around the perimeter provides room to insert a plastic T-molded strip. “This provides a tough bumper and also makes the base appear much thicker than it actually is. What's more, the resulting inside cavity makes for a great place to mount equipment, such as an isolation transformer,” he says.

The design also is flexible enough to fine-tune the amount and location of extra ballast. “For example, take a medical device that has a fluid reservoir toward the front of the cart,” says Collins.” Merely adding weight to the back and removing it from the front by changing the thickness of the steel layers solves potential balance problems. The inside cavity also allows flexibility for mounting casters. Plate-mount casters can easily be attached anywhere and stem-mounted casters can be accommodated by cutting clearance holes in the layers of steel.”

An endoscopy assembly

Metal fabrication also played a big part in the development of an endoscopy device for minimally invasive surgeries. Okay Industries Inc. in New Britain Conn., (okayind.com) worked with a large medical-device manufacturer to develop the sub-assembly. Design requirements for the device specified a low-cost articulating tube that could be controlled by guide wires during surgery. The articulating tube, also known as the A-joint, would let surgeons insert different instruments inside the tube while performing the procedure.

Manufacturing the A-joint is challenging because of the complex laser welding required and tight tolerances, says Okay. The company's customer had been making the A-joint by laser cutting stainless-steel tubing and welding springs to the individual links. This method is cost effective for prototypes and low-volumes. But a more efficient way was needed for runs higher than 1million units per year.

So Okay engineers found a way to use a progressive die to stamp out interconnected individual flat links as a complete set. Links are joined while they are still flat by way of laser-welded springs. A forming operation then curls the whole assembly into a tube about 4-in. long and ½-in. diameter while a laser welds the tube seam and the last row of springs. The upshot is that costs are considerably reduced for high volume production using metal stampings instead of laser-cut tubing to produce articulation joints. Also, laser welding the springs in place while the links are flat simplified tooling and fixture design. In addition, laser welding and rollforming are automated. This helps ensure every part meets print specifications without complex costly secondary operations.

The company says it used its special prototyping process with the same tooling concepts, operation sequence, and grain direction of the metal that will be used in production tools to minimize costly changes later in the process. Issues addressed include component strength, cracking, surface finish, burrs or edge condition, and tolerance capabilities.